There are currently several commercial methods for determining efficiency in a liquid scintillation counting system. One widely used method is internal standardization. This technique consists of counting scintillations from radioactive events in a liquid sample to be analyzed, then adding a known amount of radioactivity in a compatible form. The sample with the increased radioactivity is then recounted. The efficiency is determined by dividing the count rate increment due to the internal standard by the known disintegration rate of the internal standard. Among the disadvantages associated with internal standardization is the requirement that each sample must be handled twice as well as the requirement that it be opened, with the possibility of alteration of counting efficiency due to spillage or evaporation. In addition, the sample is thereafter altered and cannot be recounted. Furthermore, each newly prepared standard solution must be compared to a permanent standard for continuity and each sample is subject to possible pipetting errors. Moreover, the precision of internal standardization falls off with increased color quenching, which is the internal absorption of some of the light produced by the scintillations in a colored sample solution.
An existing alternative to internal standardization is external standardization. External standardization requires, for example, an external standard of a source of gamma radiation of known radiation characteristics. Disintegrations from the liquid sample to be analyzed are first counted without the external standard. Thereafter the external standard is positioned near the liquid sample. The external standard emits gamma rays that induce scintillations by the scintillation fluor in the liquid sample. Calibration curves are constructed for each radioactive isotope serving as an external standard and for a particular scintillation fluor in each solvent system by means of a set of standard samples containing known amounts of radioactivity in various concentrations of quench covering the useful efficiency range. The count rate produced by the external standard in each sample analyzed can then be correlated with the efficiency of detection. If a low energy gamma emitter is used as an external standard, however, it is necessary to subtract the contribution of the sample isotope that falls in the energy range in which the external standard is being counted. Also, there is considerable variation in the measured count of an external standard depending upon sample vial volume and shape, volume of liquid in the sample vial, and position of the external standard with respect to the vial.
Still another method of obtaining efficiency is through the sample channels ratio method. Since quenching effects the average photon yield per disintegration, the pulse amplitude distribution varies with degree of quenching. Two counting energy windows may be positioned, relative to the pulse amplitude spectrum, such that the ratio of the net count rates in the two windows can be made to vary monotonically with the degree of quenching. A calibration curve relating the ratio of the channel net count rates to isotope counting efficiency can then be constructed. Because sample channels ratio involves the ratio of the counts in two amplitude windows, this technique becomes very unreliable for highly quenched samples, since very few pulse counts then appear in the upper channel.